MOS (metal oxide semiconductor) based X-Y scanning image sensors can provide a high-speed readout operation of a pixel signal, low power consumption, and high integration as compared with CCD (charged coupled Device) based image sensors. Since MOS-based X-Y scanning image sensors satisfy the requirement for image pickup apparatuses in terms of the size and performance thereof, MOS-based X-Y scanning image sensors are widely used for image sensors mounted in image pickup apparatuses. In addition, MOS-based X-Y scanning image sensors can provide a function of performing a random scan that enables readout of an electrical charge from any one specified pixel using a horizontal signal line and a vertical signal line.
In general, image pickup apparatuses (e.g., digital still cameras) including such an image sensor perform an image capturing operation as shown in FIG. 27. That is, when, for example, a release button is pressed, the image pickup apparatuses receive an image capturing start instruction. Thereafter, an image sensor is reset at a time 901. At that time, all of pixels of the image sensor are simultaneously reset (one-time reset). Subsequently, a first curtain of a mechanical shutter for opening a light path is open at a time 902 so that exposure to the image sensor is started (a mechanical first curtain). At time 903, a second curtain of the mechanical shutter for blocking the light path is closed so that the exposure is completed (a mechanical second curtain). A time difference Δt between the mechanical first curtain and the mechanical second curtain is defined as an exposure time. When the second curtain is closed and the exposure is completed, pixel data (electrical charge) is read out from each of the pixels of the image sensor at a time 904. That is, a captured image 905 is sequentially read out on a pixel line to pixel line basis (sensor readout). A diagram 910 illustrates the case where the time difference between the mechanical first curtain and the mechanical second curtain is large, that is, the case of a low-speed shutter. In contrast, a diagram 920 illustrates the case where the time difference between the mechanical first curtain and the mechanical second curtain is small, that is, the case of a high-speed shutter.
In such an image pickup apparatus, vibration of the mechanical focal plane shutter may occur. This vibration causes a negative impact on the image capturing (exposure) operation. That is, a curtain (a first curtain) starts running for opening a light path. When the curtain completes the opening operation, the curtain collides with a shutter plate in a shutter unit. Thus, shock vibration (vibration occurring at a time 931 shown in FIG. 28 (a first curtain shock)) occurs. This vibration causes vibration of the image sensor, and therefore, an image on the image sensor is blurred during the exposure time Δt.
To solve this problem, for example, Japanese Unexamined Patent Application Publication No. 2000-152057 describes technology in which a first curtain of the shutter operation is performed by an electronic focal plane shutter and a second curtain is performed by a mechanical focal plane shutter. That is, a reset signal that causes each of pixels of the image sensor to perform a reset operation is sequentially supplied on a pixel line to pixel line basis so that the image sensor (the electronic focal plane shutter serving as a first curtain) starts an exposure operation. The exposure operation is completed by closing a curtain (the mechanical focal plane shutter serving as a second curtain) after a predetermined exposure time has elapsed.
In general, the moving velocity of a curtain of a mechanical focal plane shutter is not constant. At a start point, the moving velocity is relatively low. At an end point, the moving velocity is relatively high since the velocity of the curtain has been increased. In addition, the moving velocity of the curtain may change in accordance with changes in temperature and moisture and a change in position thereof. Accordingly, when, using a first curtain of the electronic focal plane shutter, a reset signal is sequentially supplied to the pixel lines from the start point in the moving direction of the curtain at predetermined intervals (a constant velocity of the curtain) in order to start exposure of each pixel line, a difference occurs between the constant speed of the first curtain and the changing speed of the second curtain. Due to the difference in speed between the curtains, especially during an operation with high SS (shutter speed) which causes a slit exposure, stable exposure cannot be obtained, that is, unevenness of exposure occurs. Furthermore, since the position of the first curtain is separated from the position of the second curtain, such an optical positional shift causes an edge darkening problem of a light ray depending on the position of the exit pupil and the F no., and therefore, unevenness of exposure occurs.
Some image pickup apparatuses employ an electronic shutter for both first and second curtains. More specifically the image pickup apparatuses employ an CCD-based image sensor and perform an exposure operation, that is, the start and end operations of exposure using an electronic shutter control of the image sensor. Note that although the image pickup apparatuses include a mechanical focal plane shutter having only a curtain corresponding to the second curtain, this curtain is used simply for blocking light. In such image pickup apparatuses, total power consumption is disadvantageously increased as compared with the image pickup apparatuses using a MOS-based image sensor. In addition, in terms of power consumption of mechanical focal plane shutter, operating power for charging, that is, power required for returning the curtain to the original position is large as compared with electronic focal plane shutters. Therefore, to reduce power consumption, use of an electronic focal plane shutter is advantageous over use of a mechanical focal plane shutter.
Accordingly, the present invention provides an image pickup apparatus that achieves a first curtain using an electronic focal plane shutter and a second curtain using a mechanical focal plane shutter and that can prevent the occurrence of a difference in curtain speed between the first curtain and the second curtain or defect due to a positional shift of the first curtain from the second curtain (the occurrence of an edge darkening problem), such as unevenness of exposure, and that can reduce power consumption.